As an effective conventional technology for realizing effective use of frequency resources, there is, for example, a frequency sharing radio system. This is a method of sharing one frequency among a plurality of systems and performing communication using the frequency when the frequency is not used by other users.
A Patent Document 1 discloses a technology for realizing the frequency sharing radio system. In this Patent Document 1, a channel use state is checked by measuring interference between an own radio communication system and other radio communication systems and a channel is selected based on a result of the check to realize the frequency sharing radio system.
The following three methods can be cited as a method of using the frequency sharing radio system:
1. Frequency allocation to a single user
2. Opening a frequency band as a freely usable frequency band without allocating frequencies
3. Frequency allocation to a plurality of users with priority orders set for use
In a frequency sharing radio system indicated by 2 above, when there is an existing system that is already using a frequency, a newly established system needs to prevent interference with the existing system. In a frequency sharing system indicated by 3 above, when an existing system has a priority order concerning frequency higher than that of a new system, the new system needs to prevent interference with the existing system.    Patent Document 1: Japanese Patent Application Laid-Open No. 2002-186019